devops: add auto-rebase github action

The action is triggered with a `/rebase` comment.

Details: https://github.com/cirrus-actions/rebase

.github/workflows/rebase.yml

@@ -0,0 +1,24 @@
+on: 
+  issue_comment:
+    types: [created]
+name: Automatic Rebase
+jobs:
+  rebase:
+    name: Rebase
+    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase')
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+      with:
+        fetch-depth: 0
+    - name: Automatic Rebase
+      uses: cirrus-actions/rebase@1.2
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  # https://git.luolix.topmunity/t5/GitHub-Actions/Workflow-is-failing-if-no-job-can-be-ran-due-to-condition/m-p/38186#M3250
+  always_job:
+    name: Always run job
+    runs-on: ubuntu-latest
+    steps:
+      - name: Always run
+        run: echo "This job is used to prevent the workflow to fail when all other jobs are skipped."